More than 50% of the world lead production is derived from recycled lead. It is believed that approaching 90% of lead in lead batteries is recycled. The recycling of lead batteries involves treatment of lead battery paste in order to separate the lead from the other battery components. This may involve physical separation techniques as well as chemical techniques such as hydrometallurgical processing, electrowinning and smelting (pyrometallurgical processing).
In the most common approach, the battery paste is charged into a traditional smelting blast furnace and decomposed at elevated temperatures to give metallic lead. The paste material contains a large amount of sulfur in the form of PbSO4. Decomposition of this compound requires the use of relatively high temperatures, typically 1,100° C. or more. The pyrometallurgical processing of PbSO4 is problematic for additional steps are required to prevent the release of SO2 into the environment. High temperature processes also generate significant amounts of hazardous lead bearing fumes, dust and slag. Controlling harmful emissions is expensive, and frequently requires costly and specialist equipment. The processing of these harmful by-products is often laborious and time-consuming.
Hydrometallurgical methods have been used to fix sulphur in the battery paste in the form of soluble metal sulfates, which can be separated from the insoluble lead products generated in the treatment step. However, the collected lead products often retain a substantial amount of sulfur in the form of PbSO4. If the collected product is taken to the smelter, steps must be taken to ensure that SO2 emission is adequately dealt with.
Electrowinning processes can also be problematic as complex mixtures of chemicals are required to dissolve lead in a form suitable for treatment in an electrochemical cell. Such cells are energy intensive.
U.S. Pat. No. 4,118,219 describes a process for recycling junk lead-acid batteries. The process comprises the step of converting Pb and PbO to PbSO4 using sulfuric acid. PbO2 in the battery paste is reduced, either by calcination or through use of a chemical reducing agent, such as hydrogen peroxide, and then reacted with sulfuric acid to form PbSO4. The PbSO4 is dissolved out of the battery paste with a highly concentrated aqueous ammoniacal ammonium sulfate leaching solution. The aqueous solution is separated from the undissolved impurities, and the dissolved lead then converted to lead carbonate which is separated from the dissolved impurities and unreacted Pb and PbO. Lead carbonate is converted to PbO or Pb in a calcining furnace.
U.S. Pat. No. 4,269,810 describes a method of desulfating lead-acid battery mud by adding an aqueous solution of a treating agent, such as Na2CO3 or NaOH, to the crushed battery components. In this process, PbSO4 reacts to generate metal sulfate, such as sodium sulfate, which is dissolved in the aqueous solution, and lead compounds, for example PbCO3 and Pb(OH)2, which precipitate out. The precipitated lead compounds are recovered along with solid PbO and PbO2 by use of conventional separation techniques such as settling or centrifuging.
The present inventors have realised that current methods for the recovery of lead from battery waste require several processing steps to obtain lead in a form that is suitable for use in industry, for example for use in battery manufacture.
In many of the known lead recycling processes, the product of the recycling process contains lead in a number of different forms. The exact composition of the product can be difficult to control and difficult to predict, and can be influenced by the composition of the lead waste used. Further processing may be necessary to give a product that is suitable for use in industry.
There is a need for a process of recovering lead from lead battery paste that is simple and benign. Such a process should also be cost effective and comply with environmental legislation. There is also a need to avoid transporting lead battery waste to specialist facilities, and it would be advantageous to have a process that can be employed locally. Avoiding transport costs would increase the sustainability of any such process.